Bubble Formation And Parallel Numbering-up In Microchannels

Microchemical technology is one of the important means for the intensification of the chemical process and has attracted wide attentions due to its high mass transfer,good safety and numbering-up strategy.In this paper,experimental studies were conducted on the gas-liquid two phase flow patterns,bubble formation process and the stability and homogeneity of two phase flow in parallel microchannels with different operating conditions and channel geometry.The dynamics and mechanism for N₂ bubble formation in highly viscous glycerol-water mixtures in a flow-focusing device were observed by using a high-speed digital camera.The bubble formation process can be divided into an expansion stage and a breakup stage.The volume of the gaseous thread increases linearly with time in the two different stages,however,the growth rate in the breakup stage is always greater than that in the expansion stage.The growth rate for the evolution of the gaseous thread is controlled by the capillary number and the gas-liquid flow rate ratio.The turning point of the two stages is related to the gas-liquid flow rate ratio.Finally,the volume of generated bubbles in highly viscous liquids in the flow-focusing device is scaled with the capillary number and the gas-liquid flow rate ratio.The interface dynamics of bubble in highly viscous liquids in a microfluidic flow-focusing device has been studied.Two different stages are observed in the bubble breakup process:at first,the neck radius of the gaseous thread can be expressed as a power law with the remaining time;while in the final breakup stage,the neck radius of the gaseous thread decreases linearly with time.For constant gas flow rate,the exponent in the power law stage is influenced by the liquid viscosity,and the absolute value of the slope of the linear stage decreases with the increase of the viscosity.The liquid flow rate has no obvious effect on the variation of the neck radius of the gaseous thread with time in both stages.The formation process of bubbles in parallel numbering-up microchannels with different geometry was observed,the effects of operating conditions and channel geometry on the stability and homogeneity of bubble formation were investigated.The stability of bubble formation process is better with the increase of the gas flow rate,the liquid flow rate and viscosity.The stability of bubble formation with the channel geometry from high to low is:no cavity,back cavity and front cavity.The homogeneity of bubble length in the two channels increase with the decrease of the gas flow rate and liquid viscosity,increase with the increase of the liquid flow rate.The homogeneity of the bubble length with the channel geometry from high to low is:cavity in both ends,front cavity,back cavity and no cavity.